The present invention is directed to a method for preparing inorganic ion exchangers, catalysts, getters and ceramics. In particular, the present invention is directed to a method for preparing gels, including electrophoresis gels and spherules, of hydrous zirconium oxide and variations thereof. The hydrous zirconium oxide gels are prepared using an internal gelation process through the implementation of process control parameters that control the type of gel, gel shape and size, and microstructure of the material.
Hydrated oxides of many metals (such as titanium, zirconium, hafnium, tin, aluminum, lead, cerium, tungsten, magnesium, manganese, etc.), acidic salts of polyvalent metals (phosphates, tungstates, antimonates, molybdates, tellurates, selenates, silicates, vanadates and hexacyanoferrates of elements such as ammonium, titanium, zirconium, hafnium, tin, lead, etc.), and heteropoly acid salts (ammonium molybdophosphate, ammonium phosphotungstate, ammonium molybdosilicate, ammonium tungstoarsenate, titanium phosphosilicate, etc.) are very effective inorganic ion exchange materials. Because inorganic ion exchangers are stable in high radiation fields, they are especially important in the removal of radionuclides from waste streams. They have high selectivities and efficiencies for separating and removing fission products (e.g., cesium, europium, cerium, ruthenium, zirconium, and strontium), actinides, and other elements (such as silver, lead, mercury, nickel, zinc, chromium, and fluoride) from aqueous waste streams. Most of these materials are also compatible with the matrices used for long term waste storage such as in glass, phosphate or grout. Certain metal oxides, such as iron oxide and titanium oxide, are known to be effective for use in the photocatalytic decomposition of various hazardous organics and for many other catalytic purposes. Also, many metal oxides are known to be very effective as getters in removing volatile fission products from off-gas streams over a broad range of temperature. As used herein, the term xe2x80x9cgettersxe2x80x9d is meant to include any material capable of trapping another material within the getter material. For example, quartz wool (SiO2) is used to remove volatile radioactive cesium from the off-gas stream of gas cooled nuclear reactors in Great Britain.
Inorganic exchangers and sorbents, such as hydrous zirconium oxide, are only commercially available as pure material in powder or granular form. These fine powders and granular particles are not readily adaptable to continuous processing, such as column chromatography. They have poor hydrodynamic properties. Some of these powders are also made as pellets by using binding materials; however, the binders tend to lessen the number of exchange sites that are available for use. The binders also tend to block pores and passageways to the exchange sites within the structures and can adversely affect the loading and kinetic behavior of the exchangers.
Another disadvantage of many of the powders, granular material, and pellets is lack of sorbent reproducibility of the inorganic ion exchangers. These materials are prepared in batch processes in which chemical and physical gradients can occur that cause variances in the crystal morphology and compositions of the products. Also, the granular material is not very stable and tends to powder or erode, causing problems in column operations. Pelletized hydrous zirconium oxide that is held together by binding material can be used in columns; however, the loading capacity of this material is lower. Additionally, organic binders, when used to make the pellets, are not stable when exposed to high radiation doses. Finally, resins that contain hydrous zirconium oxide particles have less capacity for loading and are not stable when exposed to high radiation.
Inorganic exchangers have also been made by taking fine particles of hydrous zirconium oxide and embedding therein organic resins or inorganic materials, such as asbestos or zeolites. However, these embedded particles suffer from the same disadvantages as the other particle and granular-based ion exchange materials.
Individuals have attempted to remedy the problems associated with powders and particles by forming gel particles. There are a number of gel forming processes used in the preparation of inorganic sorbents, catalysts, ceramics, and getters. Common to all these processes is that the constituents of the processes need to be suitable for the bonding of colloidal particles into gel structures. The gels usually are hydrous metal oxides. These processes are generally identified as xe2x80x9csol-gelxe2x80x9d processes and the chemistries are complex and path dependent. Typically, they are defined as external or internal gelation processes. In the external gelation processes, gelation reactions involve mass transfer to a second phase or fluid. By comparison, there is little or no mass transfer in the internal gelation processes.
One of the original external gel processes for the preparation of nuclear fuels was developed at Oak Ridge National Laboratories (hereinafter ORNL). It was based on the gelation of colloidal sol droplets by extracting the water from them in an immiscible alcohol. In other external gelation processes developed at various European laboratories, droplets of solutions of organic polymers or sols were chemically gelled with ammonia, usually by mass transfer of the ammonia from a surrounding gas or solution.
Making silica-alumina gel as spheres is an example of one an internal gelation process. Gel spheres were made by continuously mixing an acid solution of AlCl3 or Al2(SO4)3 with sodium silicate as drops into an immiscible organic medium. The aqueous droplets gelled while in the organic medium. The key to this process was the slow or delayed gelation of silica when the sodium silicate was acidified.
The most widely studied internal gelation processes in recent years involves the water hydrolysis of metal alkoxides. In these processes, solution temperature and pH are key parameters used in controlling hydrolysis and polymerization. However, materials made by the metal alkoxide processes typically are fine powders. Additionally, due to the complex chemistries involved and the difficulty in operating the process, it was difficult to form gel-spheres of hydrous metal oxides wherein the reaction could be controlled and the final product was predictable.
Accordingly, what is needed is a method of forming a hydrous metal oxide gel, specifically a hydrous zirconium oxide gel, wherein the gel is effective as an inorganic sorbent, catalyst, ceramic, or getter. What is also needed is a method of forming a hydrous metal oxide gel wherein the characteristics of the gel may be controlled to provide a gel which is useful for a variety of different uses. Finally, what is needed is a method of forming a hydrous metal oxide gel wherein the metal oxide gel may include other constituents which are selected to remove a variety of different materials, thereby increasing the usefulness of the metal oxide gel.
The present invention is directed to a method for preparing inorganic ion exchangers, catalysts, getters and ceramics. In particular, the present invention is directed to a method for preparing gels, including electrophoresis gels and spherules, of hydrous zirconium oxide and variations thereof. The hydrous zirconium oxide gels of the present invention are prepared using a gel-sphere, internal gelation process through the implementation of process control parameters that control the type of gel, gel shape and size, and microstructure of the gel material.
This invention is unique in that it provides a means of making ion exchangers more usable as an engineered form which can be used in large-scale column separations. The flow dynamics in column operations would be greatly enhanced by using the methods of the present invention to prepare inorganic ion exchangers as spherules. Because the spherules are stable forms and have little or no tendency of degrading under dynamic conditions, the use of inorganic ion exchangers can be greatly expanded.
Spherules of either pure hydrous zirconium oxide (HZrO) or HZrO which is embedded homogeneously throughout its matrices with fine particles of other selective ion exchange materials can be made by this invention. In some cases, gelatinous spherules of hydrous HZrO can be chemically converted to spherules of other ion exchange materials such as phosphates, silicophosphates, hexacyanoferrates, tungstates and molybdates.
One disadvantage of many of the inorganic ion exchangers that are made as powders, as granular material, or as pellets is the lack of sorbent reproducibility. These materials are prepared in batch processes in which chemical and physical gradients can occur that cause variances in the crystal morphologies and compositions of the products. When made by the present invention, these same materials are more reproducible. In some cases, the densities and porosities of an exchanger, when made as spherules, can be tailored, using the method of the present invention, by varying the chemical and physical process parameters. This allows some control over the selectivity and loading behavior of the exchanger.
The internal gelation method employed in the present invention provides a unique means of making hydrous metal oxide gel-spheres; however, the method is very different from previously described methods used to make silica-alumina gel-spheres.
The internal gelation method used in the present invention is related to the method or process that was used in the development of light water and breeder reactor spheroidal fuels at ORNL and other worldwide laboratories. The various apparatuses that have been designed to make the fuel spherules can also be used to make the hydrous zirconium oxide gels which are the subject of present invention. However, while the same preferred organic base and complexing agent may also be employed, there are significant differences in the other aspects of the process. The key to making spherules of hydrous zirconium oxide or any other metal oxide spherules lies in the formulations of the gel-forming materials and operating parameters by which the gels are formed. The formulations and the operating parameters for making gels of hydrous zirconium oxide, hydrous uranium oxide, and hydrous oxide mixtures of uranium and plutonium, and hydrous oxide mixtures of uranium and thorium are all uniquely different. The original concept for internal gelation processes which use organic bases (specifically hexamethylenetetramine (HMTA)), complexing agents (specifically urea), and metal salt solutions, was the Keuring van Electrotechnische Materialen at Arnhem (KEMA) process developed by M. E. A. Hermans et al. in the Netherlands. This process involved the production of uranium oxide spherules. One of the fuel processes which used the original idea of the KEMA process was described in U.S. Pat. No. 4,397,778 by M. H. Lloyd which emphasized the essential formulations and process parameters for making mixed oxide spherules of uranium and plutonium. Also, in U.S. Pat. No. 4,502,987 by M. H. Lloyd, et al., it was found more advantageous to heat-treat the HMTA-urea solutions by boiling for a sufficient duration and then cooling the resultant solution to about ambient temperature before admixture with a solution of metals selected from the group consisting of uranium, plutonium, thorium, and mixtures thereof, for subsequent spherule formation by passage through a formation nozzle. Heat-treated HMTA solutions of this invention expanded the use of U.S. Pat. No. 4,397,778 because these solutions allowed for the preparation of ceramic microspheres over a wider range of sphere densities in a controlled manner than previously possible with the teachings of U.S. Pat. No. 4,397,778.
The method of the present invention, while related to the prior art methods, are significantly different based upon the operating parameters used in the present invention. The method of the present invention uses optimum formulations and conditions for making hydrous zirconium oxide gels. Additionally, hydrous zirconium oxide microspheres may be made by the gel-sphere internal gelation process of the present invention. These optimum formulations and conditions create an optimum process parameter window for making the hydrous zirconium oxide spheres. The present invention is unique in that it provides a method for making ion exchangers into a more usable form rather than as a powder or granular form. Spherules of pure hydrous zirconium oxide are made by the method of the present invention. Spherules can also be made with the present invention in which very fine particles of other chemicals, metals, and biochemicals can be homogeneously dispersed throughout the matrix. Also, macroporous spherules may be made by forcing water from the spheres without shrinkage of the pores. Additionally, gelatinous spherules of hydrous zirconium oxide can be converted by chemical reactions to other ion exchanger spherules such as zirconium monohydrogen phosphate and zirconium hexacyanoferrate. Also, the hydrous zirconium oxide spherules can also be converted to other chemical forms, including, but not limited to, tungstate, molybdate, vanadate, and selenate. Furthermore, to create more surface area, spherules can be prepared containing embedded particles of material that can subsequently be dissolved and removed to create larger, interconnecting pores within the spherules. Finally, the present invention may be used to make other hydrous zirconium oxide gel shapes, such as films, fibers and slabs, which may be post-treated in the same manner as the spherules.
Once formed, the hydrous zirconium oxide gels of the present invention may be used in a variety of different processes including, but not limited to, the removal of cations, anions, or other elements from fluids and mixtures. Additionally, the gels may be used to remove radionuclides from fluids such as nuclear reactor waste waters. Finally, by including another ion exchange or sorbent particle, the gels may be used to remove selected materials from other solutions, such as the removal of gold or silver from fluids produced in the mining industry or photographic industry.
Accordingly, it is an object of the present invention to provide new methods for preparing inorganic ion exchangers and sorbents into a more useful form.
It is another object of the present invention to provide new methods for preparing more useful forms of catalysts.
Yet another object of the present invention is to provide new methods for preparing more useful forms of getters.
Another object of the present invention is to provide new methods for preparing more useful forms of ceramics.
Still another object of the present invention is to provide new methods for preparing gels for use in capillary, film or slab gel electrophoresis.
It is still another object of the present invention to provide new methods for creating more surface area in hydrous zirconium oxide gels.
Another object of the present invention is to provide new methods for forming macroporous zirconium oxide spherules.
Yet another object of the present invention is to provide new methods for converting hydrous zirconium oxide spherules to other chemical forms, including, but not limited to, phosphates, tungstate, molybdate, vanadate, and selenate.
Still another object of the present invention is to provide new methods for making ultra fine hydrous zirconium oxide particles using an electric dispersion reactor.
Another object of the present invention is to provide spherules of hydrous zirconium oxide and variations thereof that are used as inorganic ion exchangers.
It is still another object of the present invention to provide spherules of hydrous zirconium oxide and variations thereof that are used as catalysts.
Yet another object of the present invention is to provide spherules of hydrous zirconium oxide and variations thereof that are used as getters.
It is still another object of the present invention to provide ceramic precursors, such as barium zirconate or strontium zirconate, which are useful in the electronic industry, particularly in the area of computer and electronic circuitry.
Another object of the present invention is to provide new inorganic ion exchangers as microspheres that exhibit good chemical stability in acidic and basic solutions.
Still another object of the present invention is to provide new inorganic ion exchangers as microspherules that are highly selective for certain cations and anions.
Yet another object of the present invention is to provide new inorganic ion exchangers as microspherules that are compatible with final waste forms.
It is still another object of the present invention to provide new inorganic ion exchangers as microspherules that improve the flow dynamics for column operations.
These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.